This invention pertains generally to protective, surface coatings commonly known as paint coatings. Protective surface coatings are organic compositions applied to substrates, to form continuous films which are cured or otherwise hardened to provide protection as well as a decorative appearance to the substrates. Protective surface coatings ordinarily comprise an organic polymeric binder, pigments, inert fillers and other, paint coating additives. The polymeric binder functions as an organic vehicle for the pigments, inerts, and other additives and further functions as a binder for the pigments ana inert fillers in the cured or hardened paint film. Polymeric binders can be thermosetting binders based on coreactive binder components.
Powder coatings are organic coatings in dry powder form comprising essentially 100% non-volatile solids and based on a polymeric film forming binder. Powder coatings can be clear coatings without opacifying pigments or pigmented coatings containing opacifying pigments. Powder paints are finely pulverized polymeric compositions in the form of dry free-flowing fine powder which melts and flows at elevated temperatures to produce a smooth coated surface when applied to a substrate. Protective surface coatings based on powder paints are particularly useful in view of environmental compliance in that powder paints ordinarily do not contain organic solvents or emit organic compounds upon baking or heat curing.
Thermosetting acrylic polymer based powder coatings exhibit good exterior durability as well as detergent and alkali resistance. Functionally reactive acrylic copolymers can be cured or cross-linked with crosslinkers such as aliphatic diacids to obtain good film appearance. Acrylic powder coatings exhibit good surface hardness and scratch resistance but generally are less flexible than polyester powder coatings. To achieve a good film appearance with acrylic powder coatings, the melt viscosity of the acrylic copolymer binder must be low which can be obtained by reducing the molecular weight or the glass transition temperature (Tg) of the acrylic copolymer. However, reducing one physical characteristic ordinarily adversely affects physical properties and particularly storage stability where powder paints tend toil fuse upon standing. It now has been found that thermosetting acrylic powder paint compositions comprising certain glycidyl methacrylate copolymers processed in accordance with this invention provide an acrylic copolymer binder having a low molecular weight in combination with a high Tg.
The use of glycidyl functional acrylic copolymers in a liquid paint system is described in U.S. Pat. No. 2,857,354. In respect to powder coatings, U.S. Pat. No. 3,781,380, U.S. Pat. No. 3,730,930, U.S. Pat. No. 3,752,870, and U.S. Pat. No. 4,091,049 disclose the use of glycidyl acrylics in powder coatings based on relatively high molecular weight copolymers. Other patents which suggest glycidyl acrylic copolymers for powder paints include U.S. Pat. No. 3,787,520, U.S. Pat. No. 3,787,521, U.S. Pat. No. 3,876,587, U.S. Pat. No. 3,939,127, U.S. Pat. No. 3,932,367, U.S. Pat. No. 4,027,066, U.S. Pat. No. 4,042,645, U.S. Pat. No. 4,346,144 and U.S. Pat. No. 4,234,466.
It now has been found that a low molecular weight glycidyl methacrylate copolymer with a high Tg can produce an excellent cured paint film appearance by controlling the methacrylate copolymer composition within a narrow range of copolymrized ethylenic monomers, primarily methacrylate monomers, in conjunction with certain process steps for copolymerizing the monomers. Achieving an attractive cured film appearance with high Tg copolymer necessary for powder is surprising because high Tg copolymers ordinarily produce inferior cured coating surfaces exhibiting non-smooth textured surfaces known as orange peel. The resulting glycidyl methacrylate copolymer has a number average molecular weight below about 2500, a calculated Tg above about 80.degree. C., and a melt viscosity between about 150 and 400 poise. The copolymer is particularly useful as a polymeric binder in powder coatings and imparts good physical storage stability and good paint film appearance. The melt viscosity is low even though the Tg is high. The glycidyl functional copolymers are functionally reactive and coreact with carboxyl functional crosslinkers to provide thermosetting powder coatings. These and other advantages of this invention will become more apparent by referring to the detailed description of the invention and the illustrative examples.